There are various techniques being used in the electronic component industry for depositing or plating metal (e.g., copper, cobalt, gold, and nickel) onto the surfaces of electronic components. Such methods include, for example, chemical vapor deposition, metal sputtering, electroplating, and electroless metal deposition. Examples of where electroless metal deposition has been used in the electronic assembly industry are in the deposition of copper on printed circuit boards. In addition, in semiconductors, electroless deposition is used to deposit nickel on bonding packs, and in multichip modules, electroless deposition is used to deposit copper interconnects.
Electroless deposition of metal is typically carried out by first “activating” the surface of an electronic component by seeding or depositing a substance that will promote metal deposition onto the electronic component surface. However, it is possible that seeding may not be necessary. For example, on a substrate containing cobalt, nickel, rhodium, copper, or palladium, seeding may not be necessary to promote metal deposition. Seeding, when desired, can be accomplished for example through immersing the electronic component in a solution containing a seeding agent. Following activation, the electronic component is typically immersed in a solution that contains metal ions and a reducing agent. The reducing agent provides a source of electrons for the metal ions, so that metal ions near or at the surfaces of the electronic components are reduced to metal and plated out onto the electronic components.
Metal features may be formed by various techniques, depending on the desired semiconductor device. For example, the metal feature may be formed by chemical vapor deposition (“CVD”), physical vapor deposition (“PVD”), electroplating, or electroless plating. Electroless plating is used in the semiconductor industry to deposit thin, metal layers or features on the semiconductor device. Electroless plating is advantageous over other plating techniques because the plated metal is uniformly deposited and evenly coated on all surfaces, including edges and corners. In contrast to electroplating, electroless plating does not utilize electrical current to deposit the metal. However, electroless plating can only be used with particular metals because the metal must be catalytic in order to sustain the plating reaction.
Many semiconductor manufacturers are using copper in semiconductor devices. Copper wires are replacing aluminum wires because copper is more conductive and allows higher frequencies to be used with smaller line widths. Copper is also replacing aluminum as the metal in bond pads. However, the copper resistance to electromigration is degrading at high current densities used in advanced IC devices due to Cu atom migration along the top copper/dielectric interface. One of the solutions to solve copper electromigration problem is to terminate copper surface with a conductive Co alloy, which has significantly higher electromigration resistance but the sufficiently low solubility to provide good metallurgical bond between copper and metal film and at the same time sustain high electrical conductivity of copper wiring.
In most of the earlier publications including patents, the electroless cobalt/cobalt alloy electroless solutions contains sulfate or chloride ions either because they are added in together with cobalt sulfate or cobalt chloride as the source of metal ions or used in part of buffering agents (such as ammonium sulfate or ammonium chloride). However, sulfate and especially chloride ions are known to specifically and strongly bond to copper. Consequently, these ions influence any interfacial reactions, which include adsorption process. The chloride and sulfate ions will compete with metal ions on the surface, lowering the density of available adsorption sites and ultimately hindering the initiation process, a key step in the plating process.
Thus, a need still remains for an electroless deposition chemical system to provide improved stability of the process and produce same film on small isolated features as on large and dense areas of copper wiring. In view of the ever-increasing commercial competitive pressures, along with growing consumer expectations and the diminishing opportunities for meaningful product differentiation in the marketplace, it is critical that answers be found for these problems. Additionally, the need to save costs, improve efficiencies and performance, and meet competitive pressures, adds an even greater urgency to the critical necessity for finding answers to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.